As the semiconductor industry is migrating towards the technology node 16 nm or beyond, not only processing but also the metrology becomes more and more complex and challenging. Accurate and precise monitoring of the key critical features are essential for maintaining the production yields and important to help improve processing and boost the device performance. Usually many different metrology types are required and complementary for today's Fin Field-Effect Transistor (FinFET) characterizations, and none of them can meet all the measurement requirements at the same time. The most widely used metrologies today are optical critical-dimension (OCD) metrology and CD-scanning electron microscopy (CD-SEM), while the X-ray-based scatterometry metrologies, such as CD-small angle X-ray scatterometry (CD-SAXS), which utilize wavelengths that are much smaller than the features, are also under intensive evaluations in industry as the feature sizes are continuing to shrink.
The OCD metrology uses broadband light source, the wavelengths of which are normally around 200 nm to 1000 nm, to measure the average CDs, profiles, and material properties, by either ellipsometry or reflectometry, or both. It is fast, non-destructive, and gives high confidence average CDs. However, it has several drawbacks. It requires reference for accuracy verification and calibration, and it does not provide CD variation information. Even worse, the OCD results are strongly model dependent and are vulnerable to the change of the optical function properties. The high correlations of spectra response between different CD parameters also bring difficulty in OCD metrology. The CD-SEM, on the other hand, does not require the reference, and can give variation information. Modeling is not required and the optical property change does not affect the accuracy of the CD-SEM measurements. More important, CD-SEM is local surface sensitive and buried features do not correlate with the measurements. However, it is difficult for CD-SEM to measure 3D profiles, and the resolution of CD-SEM is not low enough. Many samplings of local measurements by CD-SEM are also required to give high confidence average CDs.
X-ray Scatterometry, such as CD-SAXS, is also considered as a potential metrology solution for the nanoscale features. Its principle is based on the classical x-ray scattering, which is sensitive to electron density contrast, and can avoid issues related to optical properties. The models are generally more robust than the OCD ones and parameter cross-correlations are rare. It can measure 3D profiles with high confidence accuracy and precision of average CDs. It also provides variation information, such as the line-width roughness (LWR) or line-edge roughness (LER), from the Debye-Waller type broadening of diffraction peaks. X-ray Scatterometry, however, also has its own problems. For example, the spot size of the X-ray, which is the size of the X-ray beam projected on samples, are generally large and cannot be shrunk to fit the sizes of the test keys. The test keys, on the other hand, cannot be designed to be big enough to fit the spot size of the X-ray beam due to the limitations of design rules. This causes the measurement using the X-ray Scatterometry to be very time consuming, sometimes hours or longer, or even impossible.